Field of the Invention
The present invention relates to a liquid ejection head having a plurality of ejection ports.
Description of the Related Art
The known liquid ejection methods include the thermal method and the piezoelectric method. The thermal method utilizes electro-thermal conversion elements (heaters) as energy generating elements in order to generate energy necessary for ejecting liquid. The piezoelectric method, on the other hand, utilizes piezoelectric elements (piezos) as energy generating elements. Liquid ejection heads that are based on either of these methods generally include a plurality of liquid ejection ports, a plurality of pressure chambers each of which communicates with the corresponding one of the ejection ports and a common liquid chamber for storing liquid to be supplied to the individual pressure chambers. The energy generating elements are arranged in the respective pressure chambers.
When a liquid ejection head of either of the above-described types is in operation, a pressure wave of liquid appears as the energy generating element in one of the pressure chambers is driven. The pressure wave then propagates to the remaining pressure chambers containing the respective energy generating elements by way of the common liquid chamber. Then, there can be instances where meniscus vibrations take place at the ejection ports of the remaining pressure chambers. As liquid is ejected in a condition where meniscuses are vibrating to a large extent, the ejected liquid droplets can represent variations in terms of volume, moving speed and moving direction depending on the height and the vibration velocity of the meniscuses. As the ejected liquid droplets represent variations in terms of volume, moving speed and moving direction in this way, degraded images can be recorded by the liquid ejection head because such variations entail density variations of recorded images and generation of streaky defective images. Additionally, as the meniscuses rise excessively, the ejection ports forming plane can become broadly wetted to consequently give rise to variations of liquid ejecting direction and a liquid-unejectable state.
Pressure waves as described above can be classified into two groups of pressure waves according to the difference of propagation route. One is a group of pressure waves that directly propagate from the pressure chambers where pressure waves are generated to adjacently located pressure chambers, which are referred to as directly propagating waves. The other is a group of pressure waves that propagate to the common liquid chamber and are subsequently reflected by one of the wall surfaces of the common liquid chamber to propagate to other pressure chambers, which are referred to as wall surface-reflected waves.
The magnitude of meniscus vibrations attributable to directly propagating waves depends on the distance by which pressure chambers are separated from each other. Therefore, the influence of directly propagating waves is small between two pressure chambers that are separated from each other by a large distance. Additionally, the meniscus vibrations generated by directly propagating waves survive only a short period of time after the generation of the directly propagating waves. Thus, liquid ejections by a liquid ejection head can be made to be hardly influenced by directly propagating waves by maximizing the drive time differences of the energy generating elements arranged in the respective pressure chambers that are located close to each other by adopting an energy generating element drive technique referred to as time division drive.
On the other hand, meniscus vibrations attributable to wall surface-reflected waves depend on the reflection behavior of the pressure waves. More specifically, the magnitude and the peak time of meniscus vibrations vary to a large extent depending on the starting points of the wall surface-reflected waves, the distances from the wall surface of the common liquid chamber that reflects pressure waves and the angle of the wall surface. With the above-described time division drive, it is difficult to cause the drive time difference to finely vary as a function of the position of energy generating element because of the characteristics of the drive method. Therefore, it is difficult to realize a drive situation where all the energy generating elements are made be hardly affected by wall surface-reflected waves.
Japanese Patent No. 2962726 and Japanese Patent Application Laid-Open No. H07-156403 disclose techniques for solving the problem of defective liquid ejections attributable to wall surface-reflected waves as described above. With the techniques described in Japanese Patent No. 2962726 and Japanese Patent Application Laid-Open No. H07-156403, it is possible to cause a common liquid chamber to trap air bubbles in the inside thereof and make the trapped air bubbles absorb the pressure fluctuations in the inside of the common liquid chamber by the pressure buffering effect of the air bubbles.
When utilizing the pressure buffering effect of air bubbles by means of the techniques as described in Japanese Patent No. 2962726 and Japanese Patent Application Laid-Open No. H07-156403, it is difficult to maintain the volume of the air bubbles in the common liquid chamber to a constant level for a long period of time. Then, by turn, it is difficult to maintain the pressure buffering effect on a stable basis.
It is therefore the object of the present invention to provide a liquid ejection head that can reliably and stably suppress defective ejections attributable to the pressure waves reflected by one of the wall surfaces of the common liquid chamber of the liquid ejection head.